The present invention relates to a method for controlling an electric machine, in particular for an electric or hybrid drive train of a motor vehicle, wherein the electric machine can provide a torque both at positive and at negative rotation speeds, comprising the step of performing a check with the aid of a characteristic curve as to whether a desired torque value can or may be provided by the electric machine, wherein the characteristic curve represents a limit torque with respect to the rotation speed and has a first section for positive rotation speeds and a second section for negative rotation speeds.
Moreover, the present invention relates to a control device for an electric machine, wherein the control device is designed so as to perform a method of this type. Finally, the present invention relates to a drive train for a motor vehicle having an electric machine and a control device of this type.
It is generally known in the field of motor vehicle drive technology to use an electric machine as a stand-alone drive or in combination with a drive motor of a different type (hybrid drive). In electric or hybrid motor vehicles of this type, typically electric induction machines are used as drive motors. An electronic power system is used in order to control induction machines of this type in a motor vehicle, said electronic power system comprising an AC converter that converts into AC current the DC voltage/DC current of a (high voltage) battery that is located on board the motor vehicle. The AC converter comprises generally a plurality of controllable power switches. The power switches are controlled in a pulse-width modulated manner by means of a control device, so that during the motor mode operation the electric machine generates a particular torque at a particular rotation speed of an output shaft of the electric machine.
When controlling the electric machine, it is necessary to ascertain limit torques in order to calculate the torque strategy, said limit torques providing information regarding the performance of the electric drive.
The electric machine is generally able to provide a torque both at positive and also at negative rotation speeds. This means that during the motor mode operation, the electric machine can also be used by way of example for moving the motor vehicle in the reverse direction. In some cases, the electric machine is also designed so as to operate as a generator. Insofar as this is also possible at positive and negative rotation speeds, this is referred to as a so-called 4-quadrant operation.
In order to guarantee the reliability and availability of the electric machine and/or of the drive train, it is expedient to prevent problems arising when calculating the limit torques. One problem resides in the fact that the motor mode limit torque and the generator mode limit torque are calculated at the zero crossing point of the rotation speed (any change between positive and negative rotation speed). Torque surges occur at the said zero crossing point because for example the generator mode limit torque at negative rotation speeds has a positive value and at positive rotation speeds said limit torque has a negative value. In a corresponding manner, the motor mode limit torque at negative rotation speeds has a negative value and at positive rotation speeds said limit torque has a positive value. The surge at these limits can cause problems within the torque strategy as the rotation speed changes from negative to positive, the reason being that when the rotation speed is zero it is not possible to differentiate between the motor mode operation and the generator mode operation.
On the basis of this background, it is the object of the invention to provide an improved method for controlling an electric machine, to provide an improved control device and also an improved drive train, wherein in particular the reliability and availability are improved.